1. Field of the Invention
The present invention relates generally to methods of fiber placement and fiber placement machines and, more specifically, to fiber redirect systems and associated multi-axis robotic wrists utilized with such fiber placement methods and machines.
2. State of the Art
Fiber placement is generally a technique of placing a band of fibers, such as a plurality of preimpregnated fiber tows, onto the surface of a mandrel or on an overlying work piece to form a composite structure. Fiber placement offers various advantages in forming a composite structure including the ability of placing a band of fibers at various angles, widths and lengths onto variously shaped contours and surfaces. Thus, fiber placement enables the fabrication of composite structures which exhibit complex shapes and surfaces while simultaneously enabling the band of fibers to be located and oriented in a structurally desired orientation and configuration.
Fiber placement systems generally include a supply of fiber tows, referred to herein as a creel or creel assembly. Individual tows of fiber are supplied from the creel assembly and fed to a robotic wrist which includes a placement or delivery head. The robotic wrist conventionally allows positioning of the placement head by articulating the wrist about multiple axes. For example, a multi-axis robotic wrist may allow movement about three orthogonal axes conventionally referred to as yaw, pitch and roll axes.
In feeding the fiber tows from a creel assembly to the placement, it is desirable to maintain at least a minimum level of tension within the fiber tows such that they remain relatively taut. Without such tension, the fiber tows may become twisted, displaced and/or damaged. At best, such results cause a delay in the fiber placement process and require additional maintenance of the fiber placement system by an operator thereof. However, a damaged or otherwise improperly placed fiber tow or segment thereof may ultimately result in a defective composite structure.
The fiber path of the individual tows between the creel assembly and the placement head usually includes the passing of the fiber tows around one or more redirect rollers. The redirect rollers enable the fiber tows to change directions and, also, to accommodate the changing positions and orientations of the robotic wrist as it positions the placement head for application of the fiber tows to a desired surface. In some prior art systems, redirect rollers are coupled to servo motors or other positioning devices to enable independent positioning of the redirect rollers in an attempt to define and redefine the path of the fiber tows depending on, for example, the position and orientation of the robotic wrist and its associated placement head.
However, the use of such redirect rollers has not been entirely successful in maintaining the fiber tows in a relatively taut condition. For example, as a robotic wrist positions itself at the limits of travel about its yaw, pitch and roll axes, the path of the fiber tows is conventionally lengthened, causing an additional length of material to be fed from the creel assembly for the individual tows. However, as the robotic wrist becomes relatively more retracted in its yaw, pitch and roll positions, the fiber path is conventionally shortened or contracted, causing the individual fiber tows to exhibit an amount of slack between the creel assembly and the placement head. Such slack may ultimately result in a fiber tow becoming unacceptably twisted, damaged or displaced from its individual path about the various redirect rollers.
While mechanisms, such as the above-mentioned servo motor positioning system, have been utilized in an attempt to better control the changing path of the fiber tows, such systems have been limited in their success and, further, introduce additional complexities and costs into fiber placement systems. For example, such mechanisms may require complicated computer control to correlate the movements of such a mechanism with the movements of the robotic wrist and placement head. Additionally, such mechanisms introduce additional issues of maintenance for the operator of the fiber placement equipment.
A somewhat related issue with regard to multi-axis robotic wrists includes the harnessing of numerous electrical cables or other transmission lines (e.g., hydraulic or pneumatic tubing) coupled with the various controls, sensors, motors and other actuators associated with the wrist and placement head. Again, as a multi-axis robotic wrist articulates through its various ranges of motion, such transmission lines exhibit a certain amount of slack so as to avoid overextension and tensile failure thereof. Thus, with the transmission lines exhibiting slack from time to time, depending on the position of the robotic wrist, such transmission lines may undesirably catch on a protruding object or otherwise become tangled in some manner.
In view of the shortcomings in the art, it would be advantageous to provide a fiber placement system, including a fiber redirect system and robotic wrist, which minimizes the change in length of the fiber path between, for example, a creel assembly and a placement head while accommodating the various positions, orientations and configurations the robotic wrist and placement head may assume.
Additionally, it would be advantageous to provide a fiber placement system having a fiber redirect system which does not require additional positioning mechanisms such as, for example, servo motors, with attendant computer control of the same. Rather, it would be advantageous to have such a redirect system continuously control the fiber path automatically, based on the position and orientation of the robotic wrist and placement head, without independent control of the redirect mechanisms.